One of the major problems that tissue and towel manufacturers face is the unacceptable reduction of dry strength in their products resulting from the use of an increasing percentage of recycled or secondary pulp, chemithermomechanical pulp (CTMP) and groundwood, and consequent reduction in average fiber length in paper products such as tissue and toweling in which a high degree of softness as well as dry strength is essential. Softness is a very important property in paper used for making high quality tissues and toweling, and procedure modifications or additives that achieve an increase in paper strength normally causes a decrease in paper softness or an increase in stiffness. There is therefore a need for an effective additive that will enhance paper strength without adversely affecting the softness of the paper.
European published Patent No. 0362 770A2 (Application No. 89118245.3) discloses a mixture of cationic and anionic polymers as a strengthening additive for papermaking processes, essentially for unbleached pulps containing black liquor. The mixture comprises a water-soluble, linear, cationic polymer having a reduced specific viscosity greater than 2dl/g and a charge density of 0.2 to 4 meq/g, and a water-soluble, anionic polymer having a charge density of less than 5 meq/g that is reactable in the presence of water with cationic polymer to form a polyelectrolyte complex. Combinations of cationic guar (for example, guar "derivatized" with glycidyltrimethylammonium chloride) and cationic acrylamide copolymers, with anionic polymers in addition to those already contained in the black liquor (including sodium carboxymethyl guar) are disclosed. The preferred anionic polymer content is constituted by those polymers naturally present in unbleached pulps made by either chemical or mechanical pulping.
It would be desirable to provide a a process for making paper from a bleached pulp furnish that uses a combination of cationic and anionic polymers to enhance the dry strength of the paper more efficiently than the known processes.